Trouble with Static Kits

My grade 9’s are doing static electricity at the moment, and I am experiencing some weirdness and frustration with the kits I have. Firstly I have a combination of new and old kits, but they all come with a standard collection of items – glass, ebonite and plexiglass rods, fur, felt and silk pads, and a few other odds and ends. My problem is I am finding it hard to reliably, and consistently, get a good charge, especially a positive charge.

The instructions usually suggest rubbing glass with silk to produce a positive charge on the glass. This, in my experience, never works. Last year I discovered that rubbing the glass rod with the plastic baggie the kit comes in worked well – but with the new kits I just received that doesn’t seem to work, no idea why not.

And then there is the mysterious flip-flopping plexiglass. Acrylic when rubbed with fur should become negative – but when I held it up to a negatively charged electroscope, the leaves dropped – indicating a positive charge. I tested it with a positively charged electroscope, and the leaves spread apart, confirming the positive charge. Okay, so it’s positive. But then when I tried to demo that it was positive, it suddenly had the opposite effect. It was bizarre, and my students were totally confused – as was I.

I love a teachable moment as much as the next guy, but when the message seems to be that science is arbitrary and unpredictable, I don’t think it is the right lesson to convey. I hate it when that happens.

2 thoughts on “Trouble with Static Kits

  1. Chad Orzel

    There’s a guy we sometimes get demo stuff from who has sets of larger (1″ diameter, 2′ long) rods of two different plastics, that I think he got from Home Depot. The large size means it’s really easy to get a big charge. He also suggested the plastic bags dry-cleaners use as the best thing to rub one of them.

    Of course, I never remember which is which, so every time we do the electroscope lab, I end up running through all the permutations to find the best combination. I don’t think it really matters which sign is which– take a cue from Ben Franklin and just call one positive at random. All you ever manage in static experiments is to show relative effects, so as long as you can produce two different signs, which one is _really_ positive doesn’t matter that much.

    The sign-flipping thing sounds like one of two effects: either you’ve inadvertently done charging by induction, or you got the rod too close at one point, and some charge jumped across. Was the response notably weak?

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  2. ed Post author

    Although the relative effect is important, it is useful to have at least one known reference so that predictions can be made for specific combinations of materials, using the triboelectric series.
    As for the sign flipping, it was approaching an electroscope charged by contact, and the effect was noticeable long before the rod came close enough to allow a charge to jump. And when it does jump like that there is a noticeable “twitch” in the leaves.
    I’ll keep playing with it…

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